Electronic components, such as but not limited to integrated circuits, may be mechanically and electrically connected to electrical contact pads on a printed circuit board (PCB) by several conventional methods, including a ball grid array (BGA), or a grid array (GA) that uses pins, such as a standard pin grid array (PGA), a land grid array (LGA) that use sockets with pins, a column grid array (CGA) that uses columns, a ceramic column grid array (CCGA), or other methods. A CCGA is a commonly used type of GA used in a variety of applications, including commercial and non-commercial applications.
CCGA packages use solder columns to establish electrical connections between a ceramic substrate (on which a silicon die is attached) and a printed circuit board. The solder columns are formed from high-melting temperature solder using an alloy of tin and lead (SnPb, also known as Tin-Lead solder). For example, in a common scenario, the length-to-width ratio of the solder columns used in a ceramic column grid array package is approximately 4:1, and the diameter of the solder columns is approximately 0.020 inches. These columns are first soldered to small copper interconnection points on the bottom of the ceramic device package, and thereafter attached to the printed circuit board, in both cases using conventional low melting temperature solder paste reflow techniques that are well-known.
Coefficient of thermal expansion (CTE) is a number that represents the dimensional change of a material per degree of temperature change. In a typical connection between an integrated circuit and a PCB, different CTEs will exist between the CCGA (or CGA) body material, the material of the solder joint, and the PCB material. Area array components are typically among the largest electronic devices on a circuit board due to the number of interconnects, but are limited in maximum size based on the effects of differential CTE of the associated materials. This relates to the performance and reliability of the electrical connections. The greater the differential displacements created by CTE mismatch during thermal changes, the greater concern for the mechanical and electrical integrity of the system. This is particularly true when finished assemblies must operate reliably in military and/or space environments, where wide temperature extremes are expected.
A problem with ceramic column grid array packages is that the solder columns are susceptible to failures due to temperature variations during CCGA device operation and CTE mismatch between the ceramic device package and the printed circuit board to which it is attached. The stress from the unequal expansion and contraction is absorbed primarily by the SnPb column and soft solder in the form of metallic grain structure deformation (strain), leading to formation of microcracks, which propagate and lead to column failure. Repetitive thermal cycles, which are inevitable with many electronic components, cause this column and solder joint integrity degradation and failure. The design and process objective for such an assembly is to ensure that probability of the first such failure is extremely low until well after specified service life has ended. For a given operational temperature range, the survival of CCGA device is inversely proportional to the number of expansions and contractions (caused by temperature cycling) of solder columns: with fewer cycles the projected life of the device is increased.